This invention relates to electrochemical cells and more particularly to zinc electrodes for electrochemical cells.
Development of a stabilized zinc secondary electrode is desirable for several good reasons. Zinc is a quite abundant and reasonably inexpensive metal. Its electrode discharge potential is relatively high and equivalent weight low. The zinc electrode in common construction shows good electrical conductivity and is capable, therefore, of showing good high rate discharge capability. Finally, zinc has been found to be more environmentally benign than most other metals.
Unfortunately, the slight solubility of zinc in the electrolyte produces an irreversibility on cycling. Some zinc goes into solution during charge, which is replated upon discharge. The nonuniformity of this process, however, gives rise to dendritic penetration of the separator and densification of the active material. The former causes shorting of the cell, while the latter results in gradual loss of capacity.
Much research has been devoted to stemming this zinc migration in secondaries. Some have used electrolyte additives to decrease zinc solubility. Others have added alloying compounds to the electrode itself. Although undeniable progress has been achieved, these approaches have yet to be completely satisfactory.
Robert L. Peck in U.S. Pat. No. 4,797,190, titled, "Ionic Semiconductor Materials and Applications Thereof," discloses an ionic transport membrane composite which includes from 10 to 50 weight percent of a hydrogel dispersed within a nonporous matrix material. Peck also teaches that by adding zinc or zinc oxide, as well as graphite powder or metal powder or fibers for conductance, to the composite and attaching the composite to a copper plated stainless steel grid, a zinc electrode can be produced. The electrode is resistant to zinc dendrite formation. However, carbon or graphite powder provides relately poor conduction. Metal powder or fibers, while good conductors, add substantial weight to the electrode. Similarly, standard metal grids or current collectors, such as copper plated stainless screens, add substantial weight to the electrode.
It would be desirable to produce a zinc electrode which possesses the features of the Peck electrode but which is lightweight.